Add option to show a certain field instead of the password

Fixes #12.
Make `--label` a normal option with no parameters

Fixes #13.
Refuse to run with both `--field` and `--complete`
Initial code of pw-autotype

This initial code supports some of the KeePass AutoType codes.

Fixes #1.
Move checks after option handling
Output errors using error_pw
Prefer `printf` to `echo`

In some implementations `echo` interprets the backslashes in the
parameters even without the `-e` option.